Formation of High-Conductive C Subunit Channels upon Interaction with Cyclophilin D

Inorganic polyphosphate and ion transport across biological membranes

Inorganic polyphosphate (polyP) is widely recognized for playing important roles and processes involved in energy and phosphate storage, regulation of gene expression, and calcium signaling. The less well-known role of polyP is as a direct mediator of ion transport across biological membranes. Here, we will briefly summarize current knowledge of the molecular mechanisms of how polyP can be involved in membrane ion transport. We discuss three types of mechanisms that might involve polyP: (1) formation of non-protein channel complex that includes calcium, polyP, and polyhydroxybutyrate (PHB); (2) modulation of the channel activity of PHBlated protein channels; and (3) direct effects of polyP on the function of the voltage-gated ion channels in the process that do not involve PHB.

Cyclophilin D in Mitochondrial Dysfunction: A Key Player in Neurodegeneration?

Mitochondrial dysfunction plays a pivotal role in numerous complex diseases. Understanding the molecular mechanisms by which the "powerhouse of the cell" turns into the "factory of death" is an exciting yet challenging task that can unveil new therapeutic targets. The mitochondrial matrix protein CyPD is a peptidylprolyl cis-trans isomerase involved in the regulation of the permeability transition pore (mPTP). The mPTP is a multi-conductance channel in the inner mitochondrial membrane whose dysregulated opening can ultimately lead to cell death and whose involvement in pathology has been extensively documented over the past few decades. Moreover, several mPTP-independent CyPD interactions have been identified, indicating that CyPD could be involved in the fine regulation of several biochemical pathways. To further enrich the picture, CyPD undergoes several post-translational modifications that regulate both its activity and interaction with its clients. Here, we will dissect what is currently known about CyPD and critically review the most recent literature about its involvement in neurodegenerative disorders, focusing on Alzheimer's Disease and Parkinson's Disease, supporting the notion that CyPD could serve as a promising therapeutic target for the treatment of such conditions. Notably, significant efforts have been made to develop CyPD-specific inhibitors, which hold promise for the treatment of such complex disorders.

Identity, structure, and function of the mitochondrial permeability transition pore: controversies, consensus, recent advances, and future directions

The mitochondrial permeability transition (mPT) describes a Ca2+-dependent and cyclophilin D (CypD)-facilitated increase of inner mitochondrial membrane permeability that allows diffusion of molecules up to 1.5 kDa in size. It is mediated by a non-selective channel, the mitochondrial permeability transition pore (mPTP). Sustained mPTP opening causes mitochondrial swelling, which ruptures the outer mitochondrial membrane leading to subsequent apoptotic and necrotic cell death, and is implicated in a range of pathologies. However, transient mPTP opening at various sub-conductance states may contribute several physiological roles such as alterations in mitochondrial bioenergetics and rapid Ca2+ efflux. Since its discovery decades ago, intensive efforts have been made to identify the exact pore-forming structure of the mPT. Both the adenine nucleotide translocase (ANT) and, more recently, the mitochondrial F1FO (F)-ATP synthase dimers, monomers or c-subunit ring alone have been implicated. Here we share the insights of several key investigators with different perspectives who have pioneered mPT research. We critically assess proposed models for the molecular identity of the mPTP and the mechanisms underlying its opposing roles in the life and death of cells. We provide in-depth insights into current controversies, seeking to achieve a degree of consensus that will stimulate future innovative research into the nature and role of the mPTP.

Protein folding and unfolding: proline cis-trans isomerization at the c subunits of F1 FO -ATPase might open a high conductance ion channel

The c subunits, which constitute the c-ring apparatus of the F1 FO -ATPase, could be the main components of the mitochondrial permeability transition pore (mPTP). The well-known modulator of the mPTP formation and opening is the cyclophilin D (CyPD), a peptidyl-prolyl cis-trans isomerase. On the loop, which connects the two hairpin α-helix of c subunit, is present the unique proline residue (Pro40 ) that could be a biological target of CyPD. Indeed, the proline cis-trans isomerization might provide the switch that interconverts the open/closed states of the pore by pulling out the c-ring lipid plug.